The compounds of the present invention act as an effective and selective agonist for the neuronal serotonin (5-HT) receptor located in the primary afferent nerve of enteric nervous system or central nervous system. This type of receptor is now considered as a 5-HT.sub.3 receptor. The compounds of the present invention exert their function by releasing acetylcholine from the efferent nerve ending in the digestive tracts. It is known that stimulation of the acetylcholine receptor in the digestive tracts accelerates motility of the gastrointestinal tracts and improves functional reduction of the gastrointestinal tracts [Goonman and Gilman's, The Pharmacological Basis of Therapeutics 8th edition, p.125, (1990), Pergamon Press]. It is also known that the 5-HT.sub.3 receptor is present in the presynaptic area of central nervous system and inhibits nervous activities by its stimulation [J. Neurosci., 11, 1881 (1991)].
In consequence, it is considered that a 5-HT.sub.3 receptor agonist is useful especially against gastrointestinal disorders.
Though no compound having a selective agonistic activity on the 5-HT.sub.3 receptor had been found, the inventors of the present invention reported that thiazole derivatives disclosed in WO 92/07849 possess a selective 5-HT.sub.3 receptor agonistic activity.